I dunno, looking at the growth curve in Paul Graham’s tweet I expect most of the drop in average comment quality can be attributed to the size of the user base. It’s hard to keep high-quality content the norm even in much smaller communities.

/r/nfl had a reputation for high-quality content and wasn’t a particular battleground in the Trump Wars. It’s still a good breaking news feed, and the live game threads are fun, but every post is dominated by joke comments and memes.